In the field of micro-mechanics, which will hereafter be more specifically illustrated by the field of horology, the driving in technique is very widely used, for example for fixing a wheel onto a shaft. When the material forming the part has a plastic region, which is the case of metals and alloys, the tolerances necessary for the shaft and bore can be calculated so as to obtain a tight fit without any risk of breaking the part, or deforming it. When the material does not have any, or very little, plastic region, which is the case of glass, quartz or silicon, there is a high risk of the part being broken during assembly.
These materials are used more and more frequently in horology, particularly because of their lack of sensitivity to magnetic fields, their very low thermal expansion coefficient and their density, which is much lower than that of metals or alloys. Moreover, modern machining techniques can achieve complex shapes with a high level of precision.
If a push fit is made to prevent stresses in the brittle material, there is then a risk of the part becoming detached or a moving element not being driven by the shaft. In order to overcome this drawback, one could employ the bonding technique that has long been used for securing a balance-spring onto a collet, as disclosed for example in FR Patent No. 1 447 142. U.S. Pat. No. 3,906,714 discloses an embodiment wherein the dot of adhesive both secures the balance-spring to a ring forming the collet and said ring to the balance staff.
The use of an adhesive has, however, the drawback of requiring additional machining steps to provide recesses for the adhesive, and additional step during assembly. Further, the phenomenon of aging can lead to a certain play over time.
In EP Patent No. 1 331 528, which relates to an escapement mechanism pallet for a timepiece movement, for preventing the risk of breakage when the male part of a dart is mounted in the shaped aperture formed in a fork, it is proposed fitting resilient tongues to the aperture. Moreover, in certain embodiments such as that shown in FIG. 30 of the document, the rigid zones of the aperture comprise shoulder surfaces, which position the dart in relation to the fork along a predefined orientation.
The solutions envisaged in this document are not completely satisfactory since they do not enable the male part to be precisely centred in the aperture.